or group that claimed responsibility. >> reporter: day four, president obama comes to boston and speaks at an interfaith service telling boston the country stands with it. >> the world will return to this great american city to run harder than ever and to cheer even louder for the 118th boston marathon. bet on it. >> reporter: later that day, a break in the case. the fbi releases photos and surveillance videos of these two men walking with backpacks. at 10:48 p.m., gunshots are heard on the campus of m.i.t. in massachusetts. reports say that officer sean collier was killed. after that a black mercedes was stolen leading police on a chase. throwing grenades and pipe bombs out of the windows. one suspect, tamerlan tsarnaev was killed. day five, witnesses describe the mayhem. >> we saw the explosion. we must have heard about 60 gunshots. >> reporter: 2:40 a.m. a robo call sent to watertown incidents. >> there is an active incident in watertown right now. all watertown residents should remain in their homes. >> reporter: 4:22 a.m. a suspect on the run. >> we believe this to be a terrorist.